In recent years, with the development of OA appliances and the increasingly widespread use of computers, it is becoming common for ordinary households, personal offices and office units to carry out high-resolution color photograph printing or high-resolution commercial printing of posters, pamphlets and the like, which conventionally had been done in specialized printing works, and demand is increasing for higher-quality and higher-speed printing techniques.
The printing techniques that are employed include electrophotography, electrostatic recording and electrostatic printing methods, and in commonly employed methods, a photoconductive substance is used to form an electrostatic charge image on a photosensitive body by various means, the electrostatic charge image is then developed with toner, and the toner image is transferred to a print medium such as paper and heated and pressed with a roller to fix the image and obtain a print.
The binding resins used in conventional toner for electrostatic image development have been inexpensive styrene-acrylic-based resins. However, even while demand increases for clearer images and higher gloss in high-resolution color photographic printing that is used in place of high resolution commercial printing as described above and silver-salt photography, the gloss has been insufficient with the conventional styrene-acrylic-based resins. The use of polyester resins, which have excellent gloss, is therefore increasing to meet the demand for greater gloss.
Incidentally, printers are used in a wide temperature range, from a condition in which the printer interior is at low temperature immediately after start-up and immediately before printing, to a condition in which the printer interior is at high temperature due to heat accumulation with continuous printing. Fusion of a toner to a print medium often fails to occur when the printer interior is at low temperature, while toner break-up tends to occur in a high-temperature state, and problems such as thin print spots, dropouts, uneven coloration, roller blotting, and the like, are caused.
In order to solve these problems, toners must have cold offset resistance to prevent failed fusion of the toner at low temperature, and hot offset resistance to prevent breakup of the toner at high temperature.
Examples of toners designed for both cold offset resistance and hot offset resistance include binding resins which are combinations of a crystalline polyester resin that has inferior hot offset resistance but exhibits satisfactory cold offset resistance, and an amorphous polyester resin that has inferior cold offset resistance but exhibits satisfactory hot offset resistance (Patent documents 1 to 3). In this case, however, the amorphous polyester resin and crystalline polyester resin become partially mixed, creating a problem in that it becomes impossible for satisfactory performance of each resin to be sufficiently exhibited, or a problem in that, even with satisfactory performance, it is difficult to achieve adequate supply due to the large amount of expensive crystalline resin starting material that must be used.
In some cases, a low-molecular-weight amorphous polyester resin with satisfactory cold offset resistance is used in combination with a high-molecular-weight amorphous polyester resin with satisfactory hot offset resistance (Patent document 4). However, the performance of each becomes equalized so that both are inadequate, while the binding resin tends to bleed out onto the toner surface, often resulting in poor toner storage stability.
In light of this background, there is a demand for development of a polyester resin as a binding resin that allows both cold offset resistance and hot offset resistance to be obtained.
Yet in order to obtain satisfactory cold offset resistance, for example, it is necessary to lower the glass transition point or melting temperature of the polyester resin, and this requires lowering of the average molecular weight of the resin. In order to obtain satisfactory hot offset resistance, on the other hand, it is necessary for the melt viscosity of the resin not to be lowered too much but to be suitably maintained without reduction even at high temperature, and for this reason it is necessary for the average molecular weight of the resin to be increased. Because of these contradictory requirements, such as for the average molecular weight of the resin, no polyester resin with satisfactory performance has yet been obtained despite attempts to achieve both hot offset resistance and cold offset resistance.
In addition, polyester resins pose problems in terms of their production. Generally, polyester resins are synthesized by direct polycondensation reaction between an aromatic dicarboxylic acid such as terephthalic acid or isophthalic acid, and a polyhydric alcohol. However, aromatic dicarboxylic acids have very high melting points and low solubility in different polyhydric alcohols, and therefore when direct polycondensation methods are applied, problems such as sublimation of the aromatic dicarboxylic acid and non-uniformity of the reaction system can occur, potentially resulting in inconveniences such as the following.    Sublimation of the aromatic dicarboxylic acid makes it impossible to achieve precise control of the molar ratio.    Adhesion and pooling of the sublimates on the production equipment lowers the efficiency of the heat exchanger.    The sublimates can introduce a hazard of dust explosion.    Because the reaction system is non-uniform, the monomer has differing reactivity and control of the higher-order structure of the resin is hampered.    The non-uniformity of the reaction system also interferes with high molecularization.
These problems occur whether the production system is a batch system or continuous system, and are unavoidable whenever an aromatic dicarboxylic acid such as terephthalic acid or isophthalic acid is used as the starting material.